Optical fibres generally comprise filaments of optically transparent, non-ductile, "glassy" materials, for example vitreous silica, boro-silicate, or metal fluoride glasses. To enable guided transmission to take place, the fibres require a core and a cladding. Occasionally the function of the cladding may be performed by, for example, the surrounding air. In practice, however, optical fibres employed to transmit information usually have cores and claddings.
Different types of optical fibre will have different dimensions associated with them but, generally, the outer diameter varies (with fibre type) over a range of from 60 microns to 250 microns.
When optical power is to be transmitted into or out of the end of an optical fibre, the geometric characteristics of the fibre end affect the manner in which the power is transmitted. It is usually desirable that the fibre should be provided with a flat end face which lies perpendicular to the longitudinal axis of the fibre. For instance if two fibres for use in optical communications are to be fusion joined, it is known that if either end face is not perpendicular, optical power loss can be significantly increased at the joint. Where a major part of the optical power is carried by a fibre core, this power loss can be substantial, and monomode fibres are known to be particularly susceptible as a result of their small core diameters. Typically the core diameter is about 8 microns and the overall diameter including core and cladding is in the region of 125 microns.
An acceptable fibre end face can be obtained by clamping the fibre under a controlled axial tension and striking it on its outermost glass surface with a cleaver blade. The blade scores the fibre by entering the fibre to a very small depth relative to the diameter of the fibre, and the cleaving is effected by the longitudinal tension in the fibre. The tension may be applied during, or after, the scoring. In some known cleaving tools the cleaver blade may be mounted on a lever which drops under the influence of gravity and/or a spring to bring the blade into contact with the fibre.
In our prior European patent application, publication No: EP-A-0 152 289 (British Telecommunications) a cleaver blade is mounted as part of a rotatable cleaver assembly which is balanced about its axis of rotation. The cleaver blade is attached to an arm which extends radially from the wheel and strikes the fibre upon rotation of the wheel. A stop attached to the wheel abuts against a reference surface being adjustable. Although this apparatus has found wide acceptance, and gives improved and acceptable cleaving, the accuracy of cleaving can be reduced after operation in adverse environments. For example the blade may intrude too far into the fibre due to inaccuracy in calibration arising from temperature effects. Poor cleaving can also arise from physical jogging of the apparatus during the scoring operation. It is an object of the present invention to provide an apparatus which is more robust and less subject to poor cleaving in adverse conditions when used in the field.
Another known apparatus for cleaving optical fibres is a device in which the fibre is held manually at one end on a leaf spring support, and is clamped at the other end by a pivotable jaw carrying the cleaver blade. In operation the jaw carrying the cleaver blade is moved towards the fibre on the leaf spring by operation of one hand, the other hand holding the main end of the fibre in place, and scoring is effected by the blade being lightly pressed against the fibre. The blade is yieldable by retracting into a bore against spring pressure after the score has been made, and the moving jaw is stopped eventually by resting against the leaf spring on either side of the fibre. After scoring, an axial tension is applied by bending the leaf spring to stretch the fibre over the bend of the leaf spring. Operation of the device requires considerable operator skill, since a satisfactory score depends upon the operator touching the blade against the fibre lightly and gently.
It is another object of the present invention to provide apparatus for cleaving optical fibres which is less dependent upon operator skill that previously known devices.
According to the present invention in a first aspect, there is provided cleaving apparatus comprising means for supporting a fibre to be cleaved, and a blade for scoring the fibre to a predetermined depth prior to cleaving by applying tension along the length of the fibre, in which the apparatus includes means for limiting to a required amount the distance to which the blade may intrude into the fibre by abutment of a stop member against a reference surface, means for adjusting the distance limit effected by the abutment of the stop member against the reference surface, and means for limiting to a required amount the distance to which the blade may intrude into the fibre by limiting the force which may be applied to urge the blade into the fibre during scoring.
It is particularly preferred that the stop member should be arranged in operation to rest against the surface of the fibre on that side of the fibre which is to be scored, which surface constitutes the said reference surface.
It is another preferred feature of the invention that, when the stop member is arranged to rest against the surface of the fibre to be scored, the blade protrudes towards the fibre in operation beyond the magnitude as the distance to which the blade is intended to intrude into the fibre during scoring that is to say approximately within the range 1 to 10 times the required intrusion depth. A preferred range of intrusion is the range 1 to 1.5 times the required intrusion depth. Preferably the blade protrudes beyond the stop member by a distance substantially equal to the distance the blade is intended to intrude into the fibre.
Preferably there is provided means for adjusting the position of the stop member relative to the blade, and in a particularly preferred form the stop member comprises a leaf spring which is fixed at one end and which is arranged to about the reference surface close to the fixed end of the leaf spring, the adjustment means being arranged to vary the position of the end of the leaf spring remote from the fixed end.
In general it is a preferred feature of the invention that the part of the stop member which abuts the reference surface in operation should be positioned closely adjacent the blade.
Considering now the said means for limiting the force applied to the blade, conveniently the force limiting means may comprise resilient biassing means for urging the blade towards the fibre, the biassing means being arranged to be yieldable when the force applied during scoring has reached the required limit. Conveniently the biassing means is arranged to be yieldable before the force applied during scoring has reached the required limit. It is particularly preferred that the resilient biassing means is pre-stressed so as to be unyielding during initial intrustion of the blade into the fibre.
In one convenient form, the resilient biassing means comprises a parallelogram linkage of two spaced-apart spring members allowing movement of the blade parallel to a mount against a resilient biassing force provided by the two spring members.
Although it is possible that the fibre may be tensioned separately from the apparatus to achieve the actual cleave, it will normally be arranged that the apparatus according to the invention includes means for applying tension along the length of the fibre to cleave the fibre at the scored position. Conveniently the cleaving tension is applied after scoring has been achieved.
In a particularly preferred form, the apparatus includes two hand-operated jaws adapted to be moved towards each other during operation by manual movement, the jaws being coupled to the blade and the tensioning means in such a manner that a single closing motion of the jaws effects both the scoring and tensioning operations to achieve cleaving. Preferably the single closing motion of the jaws also effects clamping of the fibre during tensioning, and effects movement of the blade away from the fibre after scoring and before cleaving.
Conveniently the apparatus includes first and second clamping means for clamping respectively the main portion and the waste portion of the fibre with the blade positioned between the two clamping means, the jaws being coupled to the clamping means in such a manner the closing of the jaws applies the said clamping means to the fibre automatically.
In a preferred arrangement, the tensioning means comprises a moveable portion of the fibre support means which is moveable by pivotting or bending relative to a main portion of the fibre support means to tension the fibre across the pivot or bend region.
Apparatus according to the invention, at least in preferred embodiments, provides a number of advantages over previously known devices.
In known apparatus for cleaving optical fibres, it has been the practice to limit the scoring depth either by controlling the distance to which the blade may intrude into the fibre by means of a depth stop, or by controlling the force which may be applied to the blade. The present invention provides, in its first aspect, control of both the distance to which the blade may intrude into the fibre, and also the force which may be applied during the scoring. This combination of two forms of control leads to increased reliability of the apparatus and reproducability of the cleaving operation.
By arranging for the stop member to rest against the surface of the fibre to be scored, the depth limitation is made more accurate than in prior devices where a stop member abutted a reference surface set by reference to a support means on which the fibre rested. The manufacturers' tolerance in an optical fibre is usually of the same order as the required score depth. Thus if a stop is set with reference to the surface on which the fibre is resting, the manufacturing error across the diameter may mean that the set stop position for the blade is incorrect by an amount which might equal the whole of the intended score depth. By setting the depth with reference to the surface being scored, a much more accurate limitation can be set on the distance to which the blade intrudes into the fibre.
Where in previous apparatus some limit has been set on the distance by which a blade protrudes from a surrounding housing, this amount has been very much greater than the required scoring depth, and protection against overscoring has been made by relying upon yielding of the blade against a resilient mounting. It is a particular advantage of the present invention that the blade can be arranged to protrude only by a distance of the same order as the distance to which the blade is intended to intrude into the fibre, thus reducing the chances of overscoring due to accidental jogging, or due to sticking of the resilient biassing means intended to provide yielding. Inaccurate cleaving often arises from movement of the blade relative to the fibre during scoring, for example along the length of the fibre or from side to side, and inaccuracies of this kind are reduced if the blade protrudes from a limiting stop member by only a minimum amount.
The preferred arrangement that the stop member abuts the reference surface closely adjacent the blade, reduces inaccuracies arising from temperature effects which produce expansion and retraction of supporting structure, and arising from misalignment and change of position of the blade due to physical shocks.
Considering the means for limiting the force applied to the blade, the preferred arrangement that the resilient biassing means is prestressed gives considerable advantage in that it reduces the time for which the blade is in contact with the fibre during scoring, and consequently reduces the chances of unwanted movement of the blade in a plane perpendicular to the main striking movement of the blade. In known devices where yielding means limit the force applied, a relatively long travel of the blade has been allowed while the required cleaving force builds up in the resilient biassing means. In the preferred form of the invention, the biassing means is prestressed so that the blade applies almost the required full force to the fibre as soon as the blade touches the fibre, with only a small amount of yielding movement before the blade reaches the full required force, and retracts to prevent further scoring.
A first main aspect of the invention has been set out, with preferred features relating to that aspect. There will now be set out further independent aspects of the invention.
In accordance with a second independent aspect of the invention, there is provided cleaving apparatus comprising means for supporting a fibre to be cleaved, a blade for scoring the fibre to a predetermined depth prior to cleaving by applying tension along the length of the fibre, and means for limiting to a required amount the distance to which the blade may intrude into the fibre by abutment of a stop member against a reference surface constituted by the surface of the fibre on that side of the fibre which is to be scored.
In accordance with a third independent aspect of the present invention, there is provided cleaving apparatus comprising means for supporting a fibre to be cleaved, a blade for scoring the fibre to a predetermined depth prior to cleaving by applying tension along the length of the fibre, and means for limiting to a required amount the force which may be applied to urge the blade into the fibre during scoring, the means for limiting force comprising resilient biassing means for urging the blade towards the fibre, the biassing means being arranged to be yieldable when the force applied during scoring has reached the required limit, the resilient biassing means being prestressed so as to be unyielding during initial intrusion of the blade into the fibre.
In accordance with a fourth independent aspect of the invention, there is provided cleaving apparatus comprising means for supporting a fibre to be cleaved, a blade for scoring the fibre to a predetermined depth, means for applying tension along the length of the fibre to cleave the fibre at the scored position, and two hand-operated jaws adapted to be moved towards each other during operaton by manual movement, the jaws being coupled to the blade and the tensioning means in such a manner that a single closing motion of the jaws effects both the scoring and the tensioning operations to achieve cleaving. Preferably the single closing motion of the jaws also effects clamping of the fibre during tensioning, and effects movement of the blade away from the fibre after scoring and before cleaving.
In accordance with a fifth independent aspect of the invention, there is provided cleaving apparatus comprising means for supporting a fibre to be cleaved, and a blade for scoring the fibre to a predetermined depth, prior to cleaving by applying tension along the length of the fibre, in which the apparatus includes means for limiting to a required amount the distance to which the blade may intrude into the fibre by abutment of a stop member against a reference surface, and means for limiting to a required amount the force which may be applied to urge the blade into the fibre during scoring, the force limiting means being adjusted to limit the force applied to the blade to an amount such that excessive scoring is prevented or inhibited in the event of imperfect operation of the distance limiting being means, and the distance limiting means being adjusted to limit the distance the blade can enter the fibre to an amount such that excessive scoring is prevented or inhibited in the event of imperfect operation of the force limiting means.
Those preferred features which have been set out with regard to the first aspect of the invention are also applicable, where not already present, in the other independent aspects of the invention which have been set out.